The present invention relates to an improved method for accelerating the freezing of propelled sea water streams or impounded masses of sea water by treatment of the sea water with surfactant or ice nucleation agents or both with the result that the ice structure is stronger than conventionally formed ice structures fabricated from brine or sea water. The advantageous treatment of a propelled continuous stream of sea water with surfactant is directly attributable to the significant reduction in surface tension of the sea water and the resulting droplet formation and breakup which results in improved heat exchange with the ambient air and, as a consequence, more rapid cooling of the sea water. This is accomplished while maintaining the necessary relatively long horizontal transport of the stream and while maintaining the relatively large volume of sea water used in the formation of engineered ice structures from brine and the like.
Ice nucleation agents cause water droplets or impounded masses of water to freeze more rapidly by prevention or minimization of supercooling. The phenomena of supercooling is detrimental because water is able to persist as a liquid at a temperature nominally lower than the normal freezing temperature of the water. Rapid freezing of sea water is important in certain applications such as the construction of load-bearing ice structures in offshore Arctic regions where such structures are employed in conjunction with hydrocarbon exploration and production and in the construction of airfields, roads, camps and the like. In these applications, sea water is used exclusively as the aqueous medium and construction is usually started as soon as the ambient air temperature is sufficiently low to cause freezing of the sea water. It is economically advantageous to be able to cause the freezing of sea water to proceed as rapidly as possible so that load-bearing structures may be constructed in a relatively short period of time as compared to the methodology heretofore used.
A method commonly employed to form ice structures involves the propelling of sea water through the air as an essentially stream of sea water and over significant horizontal distances. The volume of the continuous stream may range up to 30,000 gallons per minute from a single nozzle used to propel the salt water over the needed distance. The air, by virtue of its low temperature with respect to the nominal freezing temperature of sea water (-1.6 to -2.0 degrees C. depending on salinity), acts as a coolant. The formation of droplets and the interaction of the sea water stream/droplet spray with cooler air results in freezing of the projected droplet spray. The efficiency of freezing depends on efficient heat exchange between the sprayed droplets and air. Formation of water droplets and the size of the droplets ultimately governs freezing efficiency at any ambient air temperature less than the nominal freezing temperature of the sea water. At the spray nozzle, the bulk of the sea water is in the form of a solid stream of water having high momentum in order to cover the desired relatively large horizontal distance. In the vicinity of the nozzle, shear and turbulent forces along the periphery of the water stream initiate droplet breakup and segregation. Along the trajectory of the stream/droplet spray, wind forces and gravitational forces promote increasing droplet breakup and segregation. Maximum droplet breakup, in the absence of significant wind forces, occurs at the apogee of the stream trajectory. The surface tension of the sea water is the fundamental property which governs how soon discrete water droplets will form and their size distribution for any imposed set of ambient conditions.
Load-bearing ice structures are also commonly built by forming a berm or dike and then flooding the impounded area with sea water, the process being repeated, after freezing of the sea water, as necessary until a desired thickness of ice has formed. Ice structures which are used as the support unit for large drill rigs are themselves large. Construction may require one or more months. It is necessary, therefore, to accelerate the ice construction phase so as to allow maximum time for drilling activities prior to the onset of the Spring thaw. The more or less routine application of flooding-spraying technology in conjunction with offshore Arctic applications is described in the prior art, U.S. Pat. No. 4,048,808 being a typical example.
In the production of snow as might be practiced at a ski resort, low salinity and typically fresh water is used and the water must be sprayed initially in the form of finely divided droplets in order to form snow in contrast to ice. The ratio of water to air is significantly lower than would be the case in a typical ice construction project. Atomizing nozzles are routinely used in snow making to promote quick formation of fine drops and to tailor the water droplet size distribution to promote rapid snow formation. Snow consists of masses of ice crystals or single ice crystals with a flat or platy morphology which form when finely divided water droplets are cooled by contact with ambient air. A formal definition of snow as either a loosely coherent cluster of ice crystals or as detached single ice crystals is mentioned in U.S. Pat. No. 2,676,471. In snow making operations, all water that is sprayed must be converted to snow before contact with the ground to prevent formation of ice that would be detrimental to skiing. The primary means used to make snow is rapid atomization of a water stream into relatively fine drops in order to improve air-water heat exchange. Atomization produces a distribution of droplets with low momentum. That is to say the production of fine droplets relatively close to the atomizing nozzle is inconsistent with a requirement to propel the water over a considerable horizontal distance and the formation of load-bearing ice. In snow making this limitation is an acceptable trade-off because of the significant aspect ratio of a typical ski run (length to width) and the desire to form what approximates natural snow rather than ice, the latter being objectionable for skiing. Snow making machines are moved as necessary to provide appropriate slop coverage and it is not necessary for the equipment to project streams over relatively long distances or to use the relatively large volumes of liquid used in the formation of engineered ice structures designed for relatively high loads.
A snow making operation is intended to cover a limited radial area, with respect to the snow making apparatus, with dry snow. The manufactured snow need not have any load-bearing capability. Ice nucleation agents and expanding gas streams have been described in the prior art as means of making snow production more efficient when ambient temperatures are relatively warm, largely as a result of the formation of relatively fine droplets. U.S. Pat. Nos. 3,887,580 and 4,200,228, for example, describe the use of ice nucleation agents in the production of dry snow from fresh water.
In ice construction, where the aim is to build a substantial load-bearing structure of a relatively large dimension, dry snow is undesirable and detrimental because snow contributes to a general weakening of the manufactured structure and snow does not possess the substantial strength of ice. Thus, the criteria and the porcedures normally used in snow making are of no significant use in the production of load-bearing ice structures formed of salt water or brine.
In accordance with this invention, it has been discovered that the governing property of a high volume sea water stream is formation of water droplets varying in size from 1 to about 30 mm in diameter. These droplets freeze in the form of hailstones, which are rounded or spherical masses of ice. The interior of the frozen droplets commonly contain liquid water of high salinity consistent with finite freezing rates and thermodynamic constraints that govern the freezing of saline solutions having a true eutectic. Successful ice construction requires that the projected sprayed material which falls to the surface have a liquid content. Some droplets crush on impact releasing additional brine. The fallen material undergoes partial melting and then refreezing. Excess brine drains away from the structure by virtue of its reduced freezing temperature caused by partial evaporation during flight and by salt rejection that occurs simultaneously with freezing. On impact with the ground, the brine is released and there is some partial melting of the frozen material. The newly formed slush then refreezes upon exposure to ambient temperature air. The refreezing which occurs after impact is the phenomena that is responsible for strength development in sprayed ice.
Thus, for example, the typical internal structure of a grounded ice island consists of cyclic layers of relatively strong, hard ice overlying a layer of softer material. This layering reflects the spraying cycle where the sea water stream is sprayed for a period of time and then spraying is suspended for a period of time to allow the sprayed material to freeze more completely. Freezing occurs from the air-ice interface downward. As an ice layer forms at the interface, material underneath is insulated and freezes at a reduced rate. During the next cycle of spraying, the hard frozen of the previous cycle is contacted again with brine and it undergoes partial melting. This cycle continues for the duration of the conventional ice construction.
Burial of dry snow does not lead to ice production on any useful timescale. In ice formation, nozzle spray parameters (volumes of fluid, elevation angle and direction of stream) must be constantly adjusted to avoid production of dry snow. Treatment of a sea water stream with surfactant is advantageous because droplet breakup is encouraged by surface tension reduction and the droplet size distribution may be shifted to smaller droplet sizes without significantly impacting the horizontal distance over which sea water is projected. Application of ice nucleation agents to a sea water stream is also advantageous by minimization of supercooling that might otherwise retard the rate at which the sprayed stream/droplet material freezes during transit from the nozzle and after impact of partially frozen material. Optimum benefit can be achieved by treatment of a sea water stream with both surfactants and ice nucleation agents in order to achieve the desired drop formation at the proper point in the stream transit and to reduce the adverse effects of supercooling.
Ice construction using flooding techniques is effective because it is possible to freeze a shallow impounded mass of water. Cooling occurs at the water-air interface. An intrinsic property of water is the attainment of maximum density at a temperature slightly above its freezing temperature. This property allows for more uniform cooling of a large impounded water mass. The advantageous application of an effective ice nucleation agent in flooding operations is due to the minimization of supercooling that could result in significant amounts of brine trapped in an ice structure. Brine pockets in an engineered ice structure, formed by a spraying technique or through freezing of an impounded mass or both, contribute to a general weakening of the structure and are, thus, undesirable.
In summary, important distinctions can be made between engineering requirements for snow production and ice construction. In snow making, water is atomized to improve heat exchange efficiency with the ambient air. This requirement precludes high volumetric flow rates from a single nozzle and also limits the maximum horizontal distance that the water droplets may be discharged from a single nozzle owing to the low momentum of very fine droplets. In ice construction in accordance with this invention, nozzles are selected which form continuous water streams because the objective is to spray large volumes of water over a substantial horizontal distance where 50 to 100 meter radii or more is typical. The air-to-water heat exchange is poor in comparison to a typical snow making operation. As a further distinction, snow making employs fresh water whereas in Arctic regions, sea water is used exclusively in ice construction. Finally, snow making has as its objective, production of dry snow consisting of flat or platy clumps of ice crystals or of single ice crystals of the same morphology. The end product is insufficient as a load-bearing structure. In contrast, ice construction by either spraying or flooding techniques has as its objective production of ice with substantial load-bearing capability. Sea water spraying produces spherical hailstones which may contain entrained brine. The transformation of these hailstones to ice involves partial melting after impact and rapid refreezing. Dry snow in this context is detrimental and counterproductive to the timely completion of an engineered structure.